Transducers designed for therapeutic ultrasound applications deliver therapeutic power levels through piezoelectric ceramics such as PZT (Lead Zirconate Titanate) or through PZT/polymer composites. The transducer consists of a single piezoelectric element or multiple elements. Electrodes are connected to each piezoelectric element to generate ultrasound waves and to control wave properties such as frequency, amplitude and phase of ultrasound waves. The single element transducer has a fixed focal length that generates a fixed focal position, while the phased array (with multiple elements) transducer possesses the ability to steer the focus position by tuning each element's phase, which is called beam forming.
Therapeutic ultrasound is a minimally invasive or non-invasive method for depositing acoustic energy into tissue. The most common therapeutic application for an ultrasound transducer is to deliver focused ultrasound or High Intensity Focused Ultrasound (HIFU) to heat and destroy pathogenic tissue or to help drug delivery and release inside the body. Therefore, a therapeutic ultrasound transducer is designed to deliver acoustic energy through multiple layers of human skin, fat, muscle and soft tissues and to focus an acoustic beam at one specific zone under such layers. The precision with which the focus position is steered the energy level at the focus position are critical. Moreover, dynamic focusing of the transducer is required to track and target tumors in the moving organ, e.g. liver tumors.